1. Field of the Invention
The present invention relates to an optical information reproducing device and a method of controlling thereof, particularly to a tracking control device of a light receiving element of an optical information reproducing device for reproducing information recorded in an optical disk such as a CD (Compact Disk), a DVD (Digital Video Disk) or the like and a method of controlling thereof.
2. Description of the Background Art
In reproducing information recorded on an optical disk such as a CD or a DVD or the like, a laser beam having a predetermined wavelength is irradiated on a track (pit) recorded on the optical disk and light of the laser beam reflected from the track is detected by a pickup (light receiving element) whereby reproducing signals are formed.
The pickup includes a photoelectric conversion element which converts detected light from the optical disk into electric signals and outputs them. The photoelectric conversion element is normally constituted by four photodiodes, and the light reflected from the track is detected by each of the four photodiodes and electric signals are outputted on four signal paths as output signals from the same track.
Meanwhile, it is especially important in the optical information reproducing device that the reflected light from pit rows on the track recorded on the optical disk is detected with no deviation. That is, the track is inscribed on the optical disk at a high density and a very small width and accordingly, fine control of the pickup is necessary such that the pickup accurately detects light reflected from the track. In the following explanation control of the pickup is referred to as tracking control.
For tracking control, a method of using an auxiliary pickup for detecting exclusively only data for tracking in addition to a pick up for detecting data for information and a method of using the same pickup for detecting both data for information and data for tracking are known. There has been a method according to the latter system in which respective electric signals from two of the four signal paths, from a photoelectric conversion element included in a pickup, are added to give a phase difference between the two electric signals which is fed back to an actuator motor for controlling the pickup by which the position of the pickup is controlled. Such a method of tracking control is the most generally-used conventional method and also the device is generally known.
According to the above method, the tracking error value can be detected as a voltage level by calculating the difference between the output signal values of each pickup. Then, the tracking error value can be detected without the influence of electrical noise, or the intensity of the light reflected from the track surface.
However, according to the method where the same pickup is used for detecting both data corresponding to information and tracking data, when the optical disk is searched, that is, when the lens and pickup are shifted in the radial direction of the optical disk, a direct current offset voltage is generated between the two signals used to calculate the phase difference. That is, a physical shift is caused between the lens and pickup. Due to this physical shift, a electrical shift based on the phase difference and electrical intensity is caused between the two routes of electric signals. This electrical shift is equal to a direct current offset voltage and it is inherent in each disk depending on the depth and shapes of the pit. The offset voltage is referred to as tracking remaining error value in the following description. It should be noted that the tracking remaining error is different from the tracking error caused by deviation of the pickup from the track or the like.
The tracking remaining error is described in Japanese Patent Laid Open (Kokai) 6-325397. The tracking error signal comprises a real tracking error component (i.e., the tracking error) and a quasi-tracking error component (i.e., the tracking remaining error). In FIGS. 5 and 6 of this application, reproduced here as FIGS. 12A and 12B, the amplitude of the real tracking error component is S (S1 or S2), and the amplitude of the quasi-tracking error component is N (N1 or N2) of respective pit depths of 60 nm and 110 nm. The real component represents a relative position of a laser beam spot to the track which is a series of pits (or projections) formed on a disc. The quasi-tracking error component is caused by movement of an object lens and does not represent this relative position.
When the tracking remaining error is present, and when servo-control of tracking is conducted for a target track after finishing the search, a physical shift is caused between, for example, a target track on the disk and a target of a laser spot and fine tracking control cannot be executed. This situation originates since the circuit for detecting the tracking error cannot distinguish between the error in the input signals of the tracking error and the tracking remaining error. Accordingly, it is necessary to electrically correct the offset voltage, that is, the tracking remaining error, when the optical disk is first loaded in a reproducing device. This allows tracking control to be executed without regard to the physical shift caused between the lens and pickup.
A system of correcting the tracking remaining error has been developed, as a technology developed for reproducing CDs, in which a portion of an output signal from a photoelectric conversion element is retarded by phase-shifting. FIG. 1 is a diagram of a device used in this system. According to this device, two of four signal paths for the output signals from a photoelectric conversion elements 13a-13d, are respectively connected to delay circuits 21a and 21b where the delay (delay time) is variable. The delays of the delay circuits 21a and 21b are set by a delay control unit 31 in accordance with the detected tracking remaining error and the delays are added to the output signals from the photoelectric conversion element 13 by which the tracking remaining error is corrected. Also in FIG. 1, reference numerals 15a, 15b, 15c and 15d designate pre-amplifiers, reference numerals 20a, 20b, 20c and 20d designate equalizers for shaping the waveforms of the output signals from the photoelectric conversion element and reference numeral 23 designates a phase detector for comparing the phases of signals produced by addition using adders 22a and 22b. Further, the tracking remaining error is corrected by the following procedure before reproducing data of a disk whenever the disk is loaded.
(1) Predetermined delays are set to the delay circuits, the lens is radially shifted and the amount of the tracking remaining error is detected. PA0 (2) The optimum delays are set in accordance with the detected amount. PA0 (3) The optimum delays are set to the delay circuits, the lens is radially shifted and the amount of the tracking remaining error is detected. PA0 (4) The above operation is finished when the tracking remaining error is within a predetermined range. The above operation is repeated when the tracking remaining error is outside of the predetermined range.
Further, in procedure (1), the tracking remaining error is detected by, at first, moving the object lens of pickup by a constant amount in the radial direction of the optical disk, for example, toward the outer edge. From a maximum value and a minimum value of the error amount of the tracking remaining error provided by the movement, a first median value is calculated. Next, the object lens is moved toward the inner edge, and a second median value is calculated in the same manner. Finally, a difference between the first and second median values is calculated, and then, the tracking remaining error is detected.
In procedure (4), the predetermined range is set such that there is no influence by the tracking remaining error on tracking control.
The specification of the pit depth (pit height or projected value of pit) of a CD is for each disk in a range of .lambda./8n through .lambda./6n, where .lambda. is wavelength of laser beam and n is the refractive index of a disk. Accordingly, the tracking remaining error has a value within the range of the projected value of pit. Further, the tracking remaining error and the corrective delay have a single monotonic relationship when the pit depth of CD is less than .lambda./4n (see FIG. 2).
In the case of the above-described conventional system, it is sufficient that the delay circuits correct the tracking remaining error for each disk within the range of the projected value (.lambda./8n through .lambda./6n). That is, as shown by a relation between the pit depth and the offset voltage, the tracking remaining error in the conventional reproducing device of CDs may be corrected only using the single relationship and within a narrow range corresponding to .lambda./8n through .lambda./6n.
However, the development of an optical disk having ultra high density capable of recording and reproducing compressed image code, voice code, etc., has been rapidly progressing in recent years. The specifications with respect to methods of storing data on an optical disk in the case of DVDs is broader than that of CDs. Each disk is manufactured with the pit depth centering on .lambda./4n and having ranges deeper and shallower than .lambda./4n, and the tracking remaining error of the pit depth for each disk occurs in both the positive direction and the negative direction with respect to the center of the pit depth of .lambda./4n. Further, in the case of the specifications of a two-layer structure manufactured by bonding together two sheets of disks, the shapes of pits viewed from one direction include both projections and recesses. The positive or negative direction of the racking remaining error is reversed in accordance with projections or recesses of pits, that is, an upper layer or a lower layer of the bonded layer.
When delay circuits similar to those in CDs are realized using APFs (All Pass Filters) to be able to comply with the DVD specifications, the following problems are caused. FIGS. 3A and 3B illustrate the electric properties of a general APF. FIG. 3A shows the relation between the frequency and delay of a signal provided by an APF and FIG. 3B shows the relation between the delay and the gain of the APF. The broken line in FIG. 3A indicates the case where the delay is large and the solid line indicates the case where the delay is small. The broken line in FIG. 3B indicates the case where the cut-off frequency of the APF is high and the solid line indicates the case where the cut-off frequency is low. As illustrated, the cut-of frequency fmax of the APF is lowered in the case where the delay of the delay circuit is set large compared with the case where it is set small. The maximum delay Dmax is lowered in the case where the cut-off frequency is set high compared with the case where it is set low.
Accordingly, when the delay circuit is realized with an APF in the case of CDs, the gain of a reproducing signal of a pit having the narrowest pit width 3T (where T is the clock period) and having a high frequency of occurrence, may be reduced in DVDs, or the delay may deviate from a constant range. As a result, it becomes difficult to accurately conduct tracking control or accurately provide the reproduced signals. Further, when a delay circuit having a time constant without deterioration of its frequency characteristic is to be manufactured, the circuit will become larger or the manufacturing cost will be increased.